High performance silicon (Si) solar cells employ a primary junction where the main voltage output is generated and a secondary junction which minimizes recombination of minority carriers. In most solar cells the secondary junction is located at the back surface and has the acronym BSF for Back Surface Field. These junctions can be created by diffusion, ion implantation, or epitaxy. Ion implantation is an expensive process not generally used in manufacturing of solar cells. Epitaxy has the advantage that the thickness and doping level can be controlled as desired and are separate. However, epitaxy is expensive and slow, requiring expensive equipment and consumables. Diffusion is most often used and is the least costly; however, diffusion requires both high temperatures and long times, and it is difficult to control junction depth and carrier concentration independently. Boron is most often used as the p-dopant for the BSF, but requires even higher temperatures, in excess of 1000 degrees C. for long times, to obtain the desired deep junction with low peak doping level. This adds to the cost, the energy balance (quantity of energy needed to make the cell versus energy returned in operation), and the time needed to manufacture cells.
The cost and environmental impact of fossil fuels is becoming a greater worldwide concern. Therefore, it is desirable to have an improved solar cell to provide a more efficient source of clean energy.